1. Field of the Invention
The present invention relates to lens units to be installed on optical devices, and in particular, relates to a lens unit including a metal lens holder worked with high dimensional accuracy at low cost and a method of making the lens unit.
2. Description of the Related Art
Optical devices used for optical communication are equipped with positioned lens units each including a lens holder and a lens fixed to the lens holder.
Japanese Unexamined Patent Application Publication No. 2007-52096 discloses a lens unit that includes a lens barrel that comprises polycarbonate and lenses held by the lens barrel such that the lenses are press-fitted in the lens barrel.
Japanese Unexamined Patent Application Publication No. 10-29825 discloses a method of making a lens unit that includes a metal lens barrel. According to this method, a glass material is supplied into the lens barrel, the glass material is heated to a glass transition point and is then pressed to form a lens, and the formed lens is secured to the inside of the lens barrel. As regards a material for the lens barrel, stainless steel is selected as a metal material having a coefficient of linear expansion (hereinafter, “linear expansion coefficient”) minimally different from that of the glass material. A predetermined length piece of a stainless steel cylinder is cut off using a slicer. The glass material is press-fitted into a center hole of the piece, thus forming the lens.
As regards the lens unit disclosed in Japanese Unexamined Patent Application Publication No. 2007-52096, since the synthetic resin lens barrel is deformed when the lenses are pressed into the lens barrel, it is difficult to increase the dimensional accuracy of the entire lens unit. Furthermore, since the dimensional accuracy of the outside shape of the lens barrel is unstable, it is also difficult to increase the positioning accuracy relative to an optical device.
The lens unit disclosed in Japanese Unexamined Patent Application Publication No. 10-29825 includes the metal lens barrel. Accordingly, the dimensional accuracy of, for example, the diameter of the outer surface of the lens barrel can be increased higher than that in a synthetic resin lens barrel. Since the lens barrel is formed by slicing the metal cylinder, however, further cutting has to be performed in order to form a tapered surface in the center hole or an outer tapered surface at one end of the outer surface. Disadvantageously, the cost of production is increased.
Furthermore, formation of the lens barrel using stainless steel by cutting requires longer cutting time. Disadvantageously, the life of a cutting tool, such as a tool bit, will be shortened. The use of free-cutting stainless steel leads to relatively easy cutting but may adversely affect the environment, because the free-cutting stainless steel contains lead.